1. Field of the Invention
The present invention pertains to a method and apparatus for protecting the conductive, typically metallic, portions of gas plasma process chambers with a material which can be cleaned using gas plasma etching procedures.
2. Background of the Invention
The semiconductor industry relies on high throughput, single substrate processing reactors which can be used for a variety of different processes such as thermal chemical vapor deposition (CVD), plasma-enhanced CVD (PECVD), plasma-assisted etching, and deposition topography modification by sputtering. It is preferable to have such processing reactors be self cleaning using plasma etching techniques to avoid labor-intensive manual cleaning and loss of productivity which arise when the reactor must be opened for cleaning.
The PECVD processing reactor chambers, which contain controlled gaseous environments at reduced pressures, are generally constructed of aluminum, although specialty alloys and materials such as quartz have been used at times. Due to the broad experience of the semiconductor industry with aluminum reaction chambers, an understanding of the effect of the presence of the aluminum upon products produced in the reactors has been developed. Thus, those working in the industry are comfortable with the use of aluminum reaction chambers. However, as described in U.S. Pat. No. 4,563,367 to Sherman, plasma-assisted chemical vapor deposition processes tend to form deposits on internal surfaces of the reaction chambers so that the surface of the aluminum reaction chambers must be periodically cleaned.
This periodic cleaning can be done by disassembling the chamber and immersing the chamber parts in a wet chemical etchant bath or by mechanically cleaning the chamber parts. However, these methods are time consuming and may produce yield-decreasing particulates and contaminants. An alternative to these methods is a self-cleaning reactor system, wherein the reaction system plasma electrodes or coils are used to generate an etching plasma which does the cleaning. Typically an etching plasma generated from CF.sub.4 and O.sub.2 is used to clean the reactor chamber. However, it has been discovered that after several plasma cleaning sequences, reactor system performance frequently degrades to the point that non-uniform depositions on semiconductor wafers occur. Investigation of the possible causes of this performance degradation led to the discovery that aluminum trifluoride (AlF.sub.3) crystallized on the bare aluminum upper electrode of the reactor system during standard plasma self-cleaning. One theory of causation was that the deposits resulted from ion bombardment by active fluorine species in the high RF field which was applied between the electrodes during the plasma self-cleaning process. It was further theorized that the fluorine destroyed the natural oxide protective coating on the bare aluminum of the upper electrode and that the AlF.sub.3 deposits formed during the plasma self-cleaning cycles. Sherman concluded that use of a small, enclosed plasma generation chamber within a conventional semiconductor-processing vacuum chamber controlled certain highly active species within the generation chamber, enabling the use of increased RF power (higher plasma etching rates) without damage to the conventional semiconductor-processing vacuum chamber. Thus, higher semiconductor-processing etch rates should be possible without increasing the required frequency of wet chemical cleaning of the conventional processing vacuum chamber. The small, enclosed plasma generation chamber is constructed from a ceramic material such as aluminum oxide. He said that the conventional processing vacuum chamber could be constructed with either metallic or ceramic walls, but he stated no preference. Another attempt to reduce the formation of deposits on CVD reaction chamber walls is described in U.S. Pat. No. 4,875,989 to Davis et al. Davis et al. describe the use of a conical baffle which is in proximity to the wafer face, to direct the stream of activated species. The conical baffle is fabricated from aluminum which is anodized over its entire surface except for its base.
U.S. Pat. No. 4,960,488 to Law et al., issued Oct. 2, 1990, describes a process for cleaning a reactor chamber, both locally adjacent to the RF electrodes and also throughout the chamber and exhaust system. Preferably, a two-step continuous etch sequence is used in which the first step uses relatively high pressure, close electrode spacing and fluorocarbon gas chemistry.. The second step uses relatively lower pressure, farther electrode spacing and fluoride gas chemistry. Typically, an etch gas mixture of C.sub.2 F.sub.6 and O.sub.2 is used for the first cleaning step, and NF.sub.3 is used as the etch gas for second cleaning step. To avoid the formation of contaminant compounds, which occurs when halogen-containing plasmas contact aluminum reactor chamber walls during etch cleaning and to reduce the amount of etch cleaning necessary, the reactor chamber is designed to have spacing between elements which reduces the opportunity for active CVD species to deposit on chamber walls outside the wafer area. In addition, particular surfaces inside the process chamber are temperature controlled to reduce gas decomposition, or condensation on chamber walls and the reactor is designed to have purging gas flow to prevent deposition of CVD materials outside the wafer area.
In a further attempt to protect the aluminum reactor chamber from attack by fluorine-containing gases, Lorimer et al. developed a method of forming a corrosion-resistant protective coating on an aluminum substrate, as described in U.S. Pat. No. 5,069,938. The protective coating is formed by first forming a high purity aluminum oxide layer on an aluminum substrate and then contacting the aluminum oxide layer with one or more high purity fluorine-containing gases at elevated temperature. The aluminum oxide layer may be either a thermally formed layer or an anodically formed layer having a thickness from at least about 0.1 micrometer up to about 20 micrometers. The preferred fluorine-containing gases will comprise acid vapors. Examples of fluorine-containing gases include gaseous HF, F.sub.2, NF.sub.3, CF.sub.4, CHF.sub.3, and C.sub.2 F.sub.6. As is evidenced by the process and the description of the finished coating, the fluoride-containing gas penetrated the aluminum oxide (possibly to the aluminum surface beneath) to form fluorine-containing compounds within. The protective coating of Lorimer et al. is intended to protect the chamber walls of the processing apparatus from the chemicals used in chemical vapor deposition and etching processes. However, applicants have determined that a thermal or anodized aluminum oxide coating of 20 micrometers or less on an aluminum surface does not prevent the gradual build up of fluoride-containing compounds such as aluminum trifluoride (AlF.sub.3), ammonium fluoride (NH.sub.4 F), and aluminum oxyfluorides (AlO.sub.x F.sub.y) upon the outer surface of the coating. These compounds eventually peel off from the surface of the coating and become a source of particulate contamination.
As can be seen from the above descriptions, the semiconductor industry would find it highly desirable to have a means of preventing the build up of deposits on aluminum process reaction chambers. Build up must be prevented even when the process reactor chamber is designed to limit plasma operational areas to the minimum space possible within the reactor, with a spacing between reactor elements which reduces the opportunity for active CVD species to deposit on chamber walls outside the wafer area.
There has been heightened interest in liquid crystal displays in recent years, in particular for flat panel displays such as computer display screens, direct view and projection television screens, and navigation and communication instruments. Such liquid crystal flat panel displays utilize the kinds of materials and physical structures generally present in semiconductor devices. Many of the processes required to fabricate the liquid crystal displays are the same processes as those used to produce semiconductor devices. Thus, semiconductor process equipment is presently being modified for use in the production of such flat panel displays. For many of the process steps, plasma-assisted CVD and plasma etching is utilized. Typically the substrate to be processed is positioned on a lower, grounded platen electrode, while the RF power is applied to the upper electrode. The reactant gases are discharged in the region of the upper electrode and a plasma forms between the two electrodes. Because the rectangular flat panel display substrates are very large by comparison to silicon wafers (up to 360.times.4.50 mm flat panels compared to 200 mm maximum diameter for silicon wafers), it is impractical to make the process reaction chamber very much larger than the plasma zone. The close proximity of the reactor chamber walls to the plasma zone increases the amount of deposition on the chamber walls and increases the possibility for arcing between the platen electrode and the chamber walls.
The build up of deposition on reaction chamber walls is critically important since flat panel displays are as sensitive to particulate contamination because they include semiconductor devices with submicron geometries. The flat panel design geometries, that is, lateral dimension, are typically large, in the range of 5 to 20 .mu.m. However, the device layers, that is, vertical dimension, are thin, in the range of 0.03 to 0.3 .mu.m, so the presence of small particles can cause electrical leakage currents which can cause a pixel or row of pixels to fail. Also, considering that a display may have 1,000.times.1,000 lines, there are one million pixels being controlled by active semiconductor transistors.
Further, since the substrate for liquid crystal display panels is typically glass, an insulating material, there is an increased tendency for the plasma to expand toward the chamber walls and for arcing to occur between the platen electrode (suceptor) on which the glass substrate is supported and the aluminum walls of the process reactor chamber.
Thus, it is important in the production of semiconductor substrates, and particularly flat panel liquid crystal displays, to develop means for: 1) confining plasma exposure boundaries, thereby reducing the formation of deposits on the reactor chamber walls; 2) cleaning the reactor walls in a manner which does not cause a gradual build up of contaminants on the reactor walls; and 3) preventing arcing from the plasma-generating electrodes to conductive reactor chamber surfaces.